Neutron reflection diffusion studies

Tarek et al. [388] studied a system with some similarities to the work of Bocker et al. described earlier—a monolayer of n-tetradecyltrimethylammonium bromide. They also used explicit representations of the water molecules in a slab orientation, with the mono-layer on either side, in a molecular dynamics simulation. Their goal was to model more disordered, liquid states, so they chose two larger molecular areas, 0.45 and 0.67 nm molecule Density profiles normal to the interface were calculated and compared to neutron reflectivity data, with good agreement reported. The hydrocarbon chains were seen as highly disordered, and the diffusion was seen at both areas, with a factor of about 2.5 increase from the smaller molecular area to the larger area. They report no evidence of a tendency for the chains to aggregate into ordered islands, so perhaps this work can be seen as a realistic computer simulation depiction of a monolayer in an LE state. 

The time dependence of the early stages of polymer interdiffusion at interfaces is indicative of the diffusion process. The normal approach to study such interdiffusion by neutron reflectivity is to use an anneal/quench cycle where the sample is heated for a given time above the glass transition temperature (Tg) of the polymer, then quenched rapidly to room temperature, after which the reflectivity profile is measured. This has proved to be highly effective for a number of systems, but is difficult to apply when Tg is room temperature, or for small molecule ingress into a higher molecular weight polymer layer. 

Table 2 includes as well the results of recent diffusion studies by molecular dynamics (MD) calculations and by quasielastic neutron scattering. Both methods consider the process of self-diffusion and reflect the transport properties over diffusion paths of typically a few nanometers. In view of the satisfactory agreement with the PFG NMR data, there should be no doubt that genuine... 

Additional techniques exist for measuring fluid absorption at adhesive interfaces. Fourier transform infrared spectroscopy in the multiple internal reflection mode (FTIR-MIR) is an available technique for studying diffusion at the interface. FTlR-MlR has provided direct evidence of water accumulation, and therefore adhesive debonding, at the interface [12,13]. The technique of neutron reflectivity has also shown that the concentration of absorbed fluid can be significantly greater at the interface than in the bulk adhesive [14-18]. 

Limitations on neutron beam time mean that only selected surfactants can be investigated by OFC-NR. However, parametric and molecular structure studies have been possible with the laboratory-based method maximum bubble pressure tensiometry (MBP). This method has been shown to be reliable for C > 1 mM.2 Details of the data analysis methods and limitations of this approach have been covered in the literature. Briefly, the monomer diffusion coefficient below the cmc, D, can be measured independently by pulsed-field gradient spin-echo NMR measurements. Next, y(t) is determined by MBP and converted to F(0 with the aid of an equilibrium equation of state determined from a combination of equilibrium surface tensiometry and neutron reflection. The values of r(f) are then fitted to a diffusion-controlled adsorption model with an effective diffusion coefficient which is sensitive to the dominant adsorption mechanism 1 for... 

Early studies, which did not include many high-order reflections, revealed systematic differences between spherical-atom X-ray- and neutron-temperature factors (Coppens 1968). Though the spherical-atom approximation of the X-ray treatment is an important contributor to such discrepancies, differences in data-collection temperature (for studies at nonambient temperatures) and systematic errors due to other effects cannot be ignored. For instance, thermal diffuse scattering (TDS) is different for neutrons and X-rays. As the effect of TDS on the Bragg intensities can be mimicked by adjustment of the thermal parameters, systematic differences may occur. Furthermore, since neutron samples must be... 

Direct Synthesis reaction of, 6 395 fluoride, 21 235, 237, 239, 249 homopolyatomic cations, 17 82 ion, stereochemistry, 2 40-41, 44-45 isocyanates, preparation, 9 158 properties, 9 157 isothiocyanates, properties, 9 177 mixed valence compounds of, 10 375-381 crystal structure of, 10 376 diffuse reflectance spectrum of, 10 380 structure of Pb," ion, 10 381 nuclear magnetic shielding, 22 224 organometallic compounds, 2 82, 88, 89 oxide, neutron diffraction studies on, 8 231-233... 

Diffuse X-ray and elastic neutron scattering studies revealed the presence of a structural change accompanying the metal-insulator transition In the insulating state below 53 K a set of satellite reflections was discovered with the superlattice... 

The adsorption of furan, 2,5-dihydrofuran and tetrahydrofuran on sodium-ion exchanged faujasites with different Si/Al ratios was studied by combining temperature-programmed desorption (TPD), inelastic neutron scattering (INS), Fourier transform infrared spectroscopy (FTIR), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), quantum mechanical computations and Monte-Carlo simulations. 

There have not been many systematic studies of the degree to which the surface of a polymer is diffuse on this scale, although neutron and x-ray reflectivity measurements are sensitive enough to provide this information. What observations exist are consistent with an interface whose width is about 5... 

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